The present invention relates to end-processed structures of extra fine coaxial cables for high-speed transmission, which are suitable for applications such as liquid-crystal displays of personal computers, terminals of small-sized communications equipment and internal wiring of electronic equipment, and to methods for producing the same.
With the recent spread of various small-sized electronic equipment including personal computers, it has come to be required that a plurality of coaxial cables are wired so as to match characteristic impedance with high precision in narrow space. In order to meet such a requirement, for each of the plurality of coaxial cables, it has become necessary that an outer conductor (shielding layer) is surely grounded, and that a central conductor is securely connected to each of determinedly spaced connector terminals or a circuit of a substrate.
For that purpose, it has come to connect the cables together, using a flat cable in which the plurality of coaxial cables are arranged at determined intervals and adhered to tapes. However, the problem has arisen that the thinner coaxial cables results in lower strength of connected portions.
As a means for solving such a problem, a process is proposed which comprises exposing shielding layers in the vicinity of end portions of a plurality of coaxial cables, fixing two pair of ground bars (metal foils) to the shielding layers with solder, bending the cables taking an approximately intermediate portion between the two pairs of ground bars as a fulcrum to cut the shielding layers, thereafter removing the shielding layers together with the metal ground bar on the end portion side to expose insulating layers (dielectric layers), adhering plastic tapes to the insulating layers by melting to fix respective insulating cores at determined intervals, followed by cutting the insulating layers with a laser beam, shifting the insulating layers of the end portions in a direction to remove them to expose central conductors, cutting off end side portions including the plastic tapes, and coating end portions of the central conductors with solder (Japanese Patent Unexamined Publication No. 10-144145).
However, in such a conventional process, the metal ground bars are fixed to the exposed shielding layers with solder in the vicinity of the end portions of the plurality of coaxial cables, so that the solder flows out of the metal ground bars to the shielding layers. Moreover, the two pairs of metal ground bars are provided, and the cables are bent, taking an approximately intermediate portion between the two pairs of ground bars as a fulcrum to cut the shielding layers, followed by removing the shielding layers together with the metal ground bar on the end portion side. As shown in FIG. 5, therefore, a solder layer 6 containing the shielding layers 3 such as served wire shielding layers protrudes and remains at end faces S of the metal ground bars 5 left on the side of the plurality of coaxial cables 1. Accordingly, connection thereof to a connector becomes difficult, and the use for applications in which high voltage is applied allows current to leak between the shielding layers 3 and the central conductor 8 to cause poor insulation in some cases. The incidence of defective products has therefore amounted to a value as high as 30% to 40%.
Furthermore, the two pairs of metal ground bars are used, and of these, the one pair on the end portion side are removed and discarded together with the shielding layers, resulting in high cost.
An object of the present invention is to solve such problems of the conventional process to provide an end-processed coaxial cable structure which has no projection of shielding layers and no protrusion of solder from metal ground bars, is very smooth in end faces of metal ground bars, so that the incidence of defective products caused by the difficulty of connection thereof to a connector or occurrence of poor insulation between shielding layers and central conductors can be significantly decreased, and moreover, can be reduced in cost because only one pair of metal ground bars are used. Another object of the present invention is to provide a method for producing the same.
For attaining the above-mentioned objects, the present inventors have conducted intensive investigation. As a result, when metal ground bars are attached onto dielectric layers with shielding layers remaining thereon in the vicinity of a cut end portion of a coaxial cable assembly formed by paralleling a plurality of coaxial cables, the present inventors have discovered that the objects of the present invention is attained by using only one pair of metal ground bars and allowing cut ends of the shielding layers to exist substantially inside the metal ground bars not to project to the outside thereof, thus completing the invention.
That is to say, according to the present invention, there are provided the following end-processed coaxial cable structures and methods for producing the same:
(1) An end-processed coaxial cable structure in which at least one end of a coaxial cable assembly formed by paralleling a plurality of coaxial cables is cut, jacket layers are removed in the vicinity of a cut end portion thereof to expose shielding layers, said shielding layers are further cut to a determined length to expose dielectric layers, said exposed shielding layers are put between metal ground bars to cover them, and said metal ground bars are fixed with solder onto the shielding layers and the dielectric layers arranged at determined intervals, which is characterized in that cut ends of said shielding layers exist substantially inside the metal ground bars and are not projected to the outside thereof;
(2) The end-processed coaxial cable structure described in the above (1), wherein the length from portions at which the jacket layers are to be removed to the cut ends of the shielding layers is shorter than the width of the metal ground bars;
(3) The end-processed coaxial cable structure described in the above (1) or (2), wherein the dielectric layers are formed of a fluororesin;
(4) The end-processed coaxial cable structure described in the above (1), (2) or (3), wherein central conductors exposed by removing the dielectric layers in the vicinity of the cut end portions of said coaxial cables are overcoated with solder;
(5) The end-processed coaxial cable structure described in the above (4), wherein end portions of the central conductors are fixed and protected at determined intervals with a fixing member;
(6) The end-processed coaxial cable structure described in the above (5), wherein the fixing member is an adhesive tape;
(7) The end-processed coaxial cable structure described in any one of the above (1) to (6), wherein the plurality of coaxial cables are colored to different colors for identification;
(8) A method for producing an end-processed coaxial cable structure comprising cutting at least one end of a coaxial cable assembly formed by paralleling a plurality of coaxial cables, removing jacket layers in the vicinity of a cut end portion thereof to expose shielding layers, further cutting said shielding layers to a determined length to expose dielectric layers, putting said exposed shielding layers between metal ground bars to cover them in a state where cut ends thereof are not substantially projected from the metal ground bars, and fixing said metal ground bars with solder onto the shielding layers and the dielectric layers arranged at determined intervals;
(9) The method described in the above (8), wherein the length from portions at which the jacket layers are to be removed to the cut ends of the shielding layers is shorter than the width of the metal ground bars;
(10) The method described in the above (8) or (9), which further comprises separating the shielding layers from the dielectric layers, fixing the shielding layers to a fixing member, pulling the shielding layers fixed to the fixing member apart from the dielectric layers, and cutting the shielding layers to a determined length;
(11) The method described in any one of the above (8) to (10), which comprises cutting said shielding layers to a determined length to expose the dielectric layers, followed by paralleling said shielding layers and dielectric layers, fixing said dielectric layers with a fixing member at determined intervals, then, putting said shielding layers between the metal ground bars to cover them in a state where the cut ends thereof are not substantially projected from the metal ground bars, and fixing said metal ground bars with solder onto the shielding layers and the dielectric layers arranged at determined intervals;
(12) The method described in any one of the above (8) to (11), which further comprises irradiating the dielectric layers on the leading edge side of said metal ground bars with a laser beam to cut and remove said dielectric layers, and overcoating exposed central conductors with solder;
(13) The method described in the above (12), which further comprises fixing the dielectric layers on the leading edge side of said metal ground bars with a fixing member, irradiating an intermediate portion between said metal ground bars and said fixing member, or said fixing member with the laser beam to cut the dielectric layers, and pulling out said fixing member, thereby removing the plurality of dielectric layers at once;
(14) The method described in any one of the above (8) to (13), wherein said dielectric layers are formed of a fluororesin;
(15) The method described in any one of the above (12) to (14), wherein end portions of the central conductors are fixed and protected at determined intervals with a fixing member; and
(16) The method described in the above (10), (11), (13) or (15), wherein the fixing member is an adhesive tape.